I'm thinking the OP may be not be talking about the zone coupons that I believe the first response refers to. The "dollars off" coupons may be from amenity kits that people receive when a flight is delayed. I'm looking at one right now that is a PD964. It is good for one passenger only and the ticket must be issued by July 31, 2006. The discount is a sliding scale:

$25 off a roundtrip fare of $75-$199
$50 off a roundtrip fare of $200-299
$75 off a roundtrip fare of $300-399
$100 of a roundtrip fare of $400 or greater

Note that these are off BASE fares (that is, before taxes & fees)

These certs are void if sold, auctioned, bartered or exchanged for compensation. (As a practical matter, I am not sure how that is enforced, but it is certainly worth knowing). While the ZONE coupons are capacity controlled, I'm not at all sure that the dollars off certs are and I'm thinking that they probably are not.